Radiator Under Pressure

My bike is sitting in my basement for the winter and I've noticed that when I have started to unscrew the radiator cap that the overflow coolant line will have air and coolant go through it as the pressure is released while cap is being unscrewed.


Why am I building pressure in my system even though the bike is just sitting?  I checked the line coming from the radiator to the overflow bottle in back and I can blow through that and make bubbles in the overflow bottle.  So that line isn't clogged.


Then I found the air vent line from the overflow bottle and was able to blow through that into the overflow bottle.


Could I have a bad radiator cap?  But I thought those springs in the cap only affected when the bike was running?


What else would be causing this pressure buildup?




How much pressure is being built up? Any change in temperature or atmospheric pressure will cause some venting when opening the radiator cap.

Not too much pressure, but enough to cause some coolant to go down the overflow tube and a hiss of air to come out of the cap.


Shouldn't be much temp or pressure changes in my basement.  

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